An implementation of the modulo 97 algorithm for calculating checksum avoiding use of large integers and allow operation in unsigned integers (< 2^31). Properties of congruence relations, more specifically this, give the correctnes of algorithm.

The project started to become acquainted with Span<T> to reduce costly string manipulations when calculating the checksum. Along the way I realized that it's possible without any allocations, so the use of ReadOnlySpan<char> in this case could just as well be replaced with char[]. The iterator IbanDigitizer could then strip the ref keyword which would allow to implement IReadOnlyCollection<int> and make for an even neater API.

Lets take iban number for a Croatian bank and disect the steps.

1. Move last four to the end

   CR23015108410023612345 -> 015108410023612345CR23

2. Replace characters with digits

   015108410023612345CR23 -> 015108410023612345122723

3. Start iteration over nine digit numbers

   1. N = 0151084410, d = N % 97 = 78
   2. N = _78_0260123, d = N % 97 = 77
   3. N = _77_451227, d = N % 97 = 25
   4. N = _25_23, d = N % 97 = 1

4. If d == 1 then valid; otherwise invalid.

The implemention make us of a specialized "iterator" that perform step 1 and 2 without any allocations.